1. Technical Field
The present invention relates to a method for producing an L-amino acid by fermentation, and more specifically to genes which aid in this fermentation. These genes are useful for the improvement of L-amino acid production, for example, for production of L-threonine, L-lysine, L-histidine, L-phenylalanine, L-arginine and L-glutamic acid.
2. Background Art
Conventionally, L-amino acids are industrially produced by fermentation methods utilizing strains of microorganisms obtained from natural sources, or mutants thereof. Typically, the microorganisms are modified to enhance production yields of L-amino acids.
Many techniques to enhance production yields of L-amino acids have been reported, including transformation of microorganisms with recombinant DNA (see, for example, U.S. Pat. No. 4,278,765). Other techniques for enhancing production yields include increasing the activities of enzymes involved in amino acid biosynthesis and/or desensitizing the target enzymes to feedback inhibition by the resulting L-amino acid (see, for example, WO 95/16042 or U.S. Pat. Nos. 4,346,170, 5,661,012 and 6,040,160).
Strains useful in production of L-threonine by fermentation are known, including strains with increased activities of enzymes involved in L-threonine biosynthesis (U.S. Pat. Nos. 5,175,107, 5,661,012, 5,705,371, and 5,939,307; EP 0219027), strains resistant to chemicals such as L-threonine and its analogs (WO 01/14525A1, EP 301572 A2, U.S. Pat. No. 5,376,538), strains with target enzymes desensitized to feedback inhibition by the produced L-amino acid or its by-products (U.S. Pat. Nos. 5,175,107 and 5,661,012), and strains with inactivated threonine degradation enzymes (U.S. Pat. Nos. 5,939,307 and 6,297,031).
The known threonine-producing strain VKPM B-3996 (U.S. Pat. Nos. 5,175,107 and 5,705,371) is presently one of the best known threonine producers. For construction of the strain VKPM B-3996, several mutations and a plasmid, described below, were introduced into the parent strain E. coli K-12 (VKPM B-7). Mutant thrA gene (mutation thrA442) encodes aspartokinase homoserine dehydrogenase I, which is resistant to feedback inhibition by threonine. Mutant ilvA gene (mutation ilvA442) encodes threonine deaminase having decreased activity which results in a decreased rate of isoleucine biosynthesis and to a leaky phenotype of isoleucine starvation. In bacteria containing the ilvA442 mutation, transcription of the thrABC operon is not repressed by isoleucine, and therefore is very efficient for threonine production. Inactivation of the tdh gene encoding threonine dehydrogenase results in prevention of threonine degradation. The genetic determinant of saccharose assimilation (scrKYABR genes) was transferred to said strain. To increase expression of the genes controlling threonine biosynthesis, plasmid pVIC40 containing the mutant threonine operon thrA442BC was introduced in the intermediate strain TDH6. The amount of L-threonine accumulated during fermentation of the strain can be up to 85 g/l.
By optimizing the main biosynthetic pathway of a desired compound, further improvement of L-amino acid producing strains can be accomplished via supplementation of the bacterium with increasing amounts of sugars as a carbon source, for example, glucose. Despite the efficiency of glucose transport by PTS, access to the carbon source in a highly productive strain still may be insufficient.
It is known that active transport of sugars and other metabolites into bacterial cells is accomplished by several transport systems. Among these, the XylE protein from E. coli is a D-xylose permease, one of two systems in E. coli responsible for the uptake of D-xylose; the other being the ATP-dependent ABC transporter XylFGH. The cloned xylE gene has been shown to complement xylE mutants in vivo (Davis, E. O. and Henderson, P. J., J. Biol. Chem., 262(29); 13928-32 (1987)). The XylE-mediated transport in whole cells is inhibited by protonophores and elicits an alkaline pH change (Lam, V. M. et al, J. Bacteriol. 143(1); 396-402 (1980)). Experiments using xylE and xylF mutants have established that XylE protein has a KM of 63-169 μM for D-xylose (Sumiya. M. et al, Receptors Channels, 3(2); 117-28 (1995)). The XylE protein is a member of the major facilitator superfamily (MFS) of transporters (Griffith, J. K. et al, Curr. Opin. Cell Biol. 4(4); 684-95 (1992)) and appears to function as a D-xylose/proton symporter. The xylE gene probably constitutes a monocistronic operon whose expression is inducible by D-xylose. Imported xylose is catabolized to xylulose-5-phosphate by the action of the XylA (xylose isomerase) and XylB (xylulokinase) enzymes. Under appropriate conditions, the xylose isomerase encoded by the xylA gene also efficiently catalyzes the conversion of D-glucose to D-fructose (Wovcha, M. G. et al, Appl Environ Microbiol. 45(4): 1402-4 (1983)).
However, there has been no report to date of using a bacterium of the Enterobacteriaceae family having an enhanced activity of D-xylose permease for increasing the production of L-amino acids.